Comments

Update build_rev
This includes several other changes required for this, because our build_rev is so out of date:
* Update clang_rev, build_overrides_rev, buildtools_revision
* Update the sysroot install command in DEPS
* Update breakpad_rev and its build invocation
* Update gtest_rev and add a gtest BUILD.gn since it disappeared from build_overrides
Change-Id: I0836ab4026d3d1ac3e903f31f9e499d510040d0c
Reviewed-on: https://chromium-review.googlesource.com/c/1366282
Reviewed-by: Nico Weber <thakis@chromium.org>